[clang] [clang] Add `intrin0.h` header to mimic `intrin0.h` used by MSVC STL for clang-cl (PR #75711)

[Fangrui Song via cfe-commits]

================
@@ -0,0 +1,26 @@
+/* ===-------- vadefs.h ---------------------------------------------------===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+/* Only include this if we are aiming for MSVC compatibility. */
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MaskRay wrote:

Newer lib/Headers changes prefer `//`-style comments. Do we need `/* */`?

https://github.com/llvm/llvm-project/pull/75711